An accelerometer sensor of this type comprises a mounting base with a barrel having a central bore receiving a fastening element designed to fix the accelerometer sensor onto a support element. The central bore comprises a spacer gripping the fastening element to center it in the central bore. An encapsulation of plastic material is overmolded around an outer wall of the mounting base.
In the following text, the accelerometer sensor is described as a knock sensor with a fastening element in the form of a specially designed screw. This is purely for the purposes of illustration and is non-limiting, since the accelerometer sensor and the fastening element may be of other types.
There is a known way of fastening a knock sensor to the crankcase or cylinder head of the internal combustion engine of a motor vehicle, by using a fastening element in the form of a screw passing through the central bore of the sensor mounting base. A knock sensor is, notably, described in FR 2 821 432 A1.
It is possible, for example, to use an M8 screw with the application of a tightening torque of between 15 Nm and 30 Nm.
Many motor manufacturers have standardized the tightening torque, allowing for a breaking stress of 33 Nm. A tightening torque of 36 Nm+/−2.5 Nm has been recommended for a grade 10.9 M8 screw. A tightening torque of 15 Nm+/−1 Nm has also been specified for a grade 10.9 M6 screw.
However, an M6 screw tightened at 15 Nm has a compressive force almost equal to an M8 screw tightened at 20 Nm, and is therefore highly suitable for mounting a knock sensor. The present tendency is therefore to use a grade 10.9 M6 screw instead of an M8 screw to mount the sensor on the crankcase. In this case, a centering spacer must be inserted into the central bore in order to grip and center the M6 screw in place of the M8 screw.
If a grade 10.9 M6 screw is used to mount the knock sensor with a centering spacer, then in all cases, that is to say regardless of whether or not the sensor is delivered with an M6 screw, the sensor must be assembled with its spacer during manufacture and must then be delivered in this assembled state.
This gives rise to serious quality control risks, since it is necessary to ensure that the spacer remains present after the assembly operation as it undergoes the operations of storage, packaging and transport and finally the various manipulations of the fitter when the sensor is mounted on the engine crankcase.
In view of these risks of loss, it is also necessary to provide a quick method of checking for the presence of the spacer, for example by giving the spacer a color for rapid visual identification. This results in additional costs for the manufacture of the knock sensor, while only slightly reducing the probability of the presence of a fault, the main faults being the loss of the centering spacer or its incorrect positioning.
For un accelerometer sensor having a mounting base comprising a central bore which receives a centering spacer of a fastening element designed to fix the accelerometer sensor onto a support element, the problem to be resolved by the present invention is that of permanently holding the spacer in position in the central bore of the sensor mounting base before the sensor is fixed to the support element, without the addition of an auxiliary element for keeping the spacer in the central bore.